1. Field of the Invention
This invention relates generally to a system and method for controlling an anode exhaust gas bleed and, more particularly, to a system and method for controlling an anode exhaust gas bleed during power up-transients and cathode pulsing.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air coming out of the stack is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
For automotive applications, it typically takes about 400 fuel cells to provide the desired power. Because so many fuel cells are required for the stack in automotive fuel cell system designs, the stack is sometimes split into two sub-stacks each including about 200 fuel cells because it is difficult to effectively provide an equal flow of hydrogen gas through so many fuel cells in parallel.
It has been proposed in the art to provide stack order switching or anode flow shifting in a fuel cell system that employs split stacks. Particularly, suitable valves and plumbing in the system are provided so that during flow shifting the anode gas exiting a first sub-stack is sent to the anode side of a second sub-stack, and the anode gas exiting the second sub-stack is sent to the anode side of the first sub-stack in a cyclical manner. During an anode exhaust gas bleed, the anode gas exiting the second sub-stack is sent to the system exhaust.
The MEAs are porous and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, referred to in the industry as nitrogen cross-over. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases beyond a certain percentage, the fuel cell stack becomes unstable and may fail. It is known in the art to provide a bleed valve to remove nitrogen from the anode side of the stack.
The anode exhaust gas that is periodically bled typically includes a considerable amount of hydrogen. Because the hydrogen will mix with air if it is vented to the environment, a potential combustible mixture may occur that provides obvious safety concerns. It is known in the art to direct the bled gas to a combustor to burn most or all of the hydrogen therein before the bled gas is exhausted to the environment. However, the combustor adds a significant cost, weight and complexity to the fuel cell system, which is undesirable.
It is also known in the art to eliminate the combustor and directly mix the bled gas with the cathode exhaust gas. If the bled gas is directly mixed with the cathode exhaust gas without control, the amount of hydrogen in the bled gas is unknown. A hydrogen concentration sensor can be provided in the cathode exhaust gas line after the mixing point with the bled gas to detect the concentration of hydrogen. The hydrogen concentration sensor would provide a signal to the controller during the bleed indicative of the concentration of hydrogen in the mixed exhaust gas. If the concentration of hydrogen was too high, the controller would increase the speed of the compressor to provide more cathode exhaust air to lower the concentration of hydrogen. If the compressor was unable to effectively keep the concentration of hydrogen below the safe limit for the stack load, then the controller would have to close the bleed valve or reduce the anode pressure. However, the hydrogen sensor would have to be inexpensive and be able to withstand the humidity of the exhaust gas. Currently, known hydrogen concentration sensors are unable to provide these requirements.
Algorithms are typically employed to estimate the concentration of nitrogen in the anode side of the stack using several input parameters based on the operating conditions of the system, and trigger an anode exhaust gas bleed when the estimated nitrogen concentration reaches a predetermined level. During an anode gas bleed, the pressure is controlled across the bleed control valve.
Controlling the hydrogen concentration in the system exhaust during anode exhaust gas bleeding is particularly troublesome during power up-transients of the stack and cathode pulsing. Before a power up-transient or cathode pulsing, the anode exhaust gas is typically bled in short pulses into the cathode exhaust gas upstream from a cathode by-pass valve that allows the cathode air to by-pass the fuel cell stack. At low current density, the cathode exhaust gas flow is not sufficient to dilute the anode exhaust gas hydrogen to be below a desired concentration, and thus, air is added to the cathode exhaust from the compressor through the by-pass valve. During power up-transients or cathode pulsing, air is redirected from the by-pass valve to the cathode inlet so that the maximum amount of air is sent to the stack for providing the power up-transient. During this time period, an anode bleed may be commanded to reduce the nitrogen concentration, where the cathode exhaust air may not be sufficient to dilute the hydrogen. Further, the compressor is not able to provide additional air through the by-pass valve because its capacity is necessary for meeting the power up-transient.